We report a blending mechanism of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and PS homopolymer (homoPS) at the air/water interface. Our blending mechanism is completely different from the well-known "wet-dry brush theory" for bulk blends; regardless of the size of homoPS, the domain size increased and the morphology changed without macrophase separation, whereas the homoPS of small molecular weight (MW) leads to a transition after blending into the block copolymer domains, and the large MW homoPS is phase-separated in bulk. The difference in blending mechanism at the interface is attributed to adsorption kinetics at a water/spreading solvent interface. Upon spreading, PS-b-PEO is rapidly adsorbed to the water/spreading solvent interface and forms domain first, and then homoPS accumulates on them as the solvent completely evaporates. On the basis of our proposed mechanism, we demonstrate that rapid PS-b-PEO adsorption is crucial to determine the final morphology of the blends. We additionally found that spreading preformed self-assemblies of the blends slowed down the adsorption, causing them to behave similar to bulk blends, following the "wet-dry brush theory". This new mechanism provides useful information for various block copolymer-homopolymer blending systems with large fluid/fluid interfaces such as emulsions and foams.